Martensitic transformation (MT) is a typical first order structural transition which proceeds by cooperative motion of atoms without diffusion. MT is basically important because it is typical phenomena of solid-state transformation. Furthermore, MT is technologically important because MT is applied for many practical steels and also for shape memory materials. MT is known to be influenced by external field such as temperature, stress and magnetic field. In this study, the effect of magnetic field was especially investigated and new functional materials, which work under magnetic fields, were invented.(1) Effects of magnetic field on martensitic transformation temperatureBy examining many iron-based alloys, we found the transformation temperature M_s under magnetic field is expressed as following : △G(M_s)-△G(M_s') = -△M(M_s')・H_c - (1/2)・χ^p_<hf>・H_c^2 + ε_0・(∂ωl ∂H)・H_c・B where △G(M_s) and △G(M_s') represent the difference in Gibbs chemical free energy between the parent and martensit
… Moree phases, △G(M_s') the difference in spontaneous magnetization between the parent and martensitic states at M_s', χ^p_<hf> the high magnetic field susceptibility in the parent phase, ε_0 the volume change associated with martensitic transformation, ω the parent forced volume magnetostriction and B the parent bulk modulus.(2)Magnetoelastic transformationAn ausaged Fe-Ni-Co-Ti alloy exhibits transformation-induced psudoelasticity above it's a_f temperature. Considering the relation between temperature and stress in thermoelastic MT, we predicted that the Fe-Ni-Co-Ti alloy will exhibit magnetoelastic MT (martensites are induced only while a magnetic field is applied and are transformed back to the parent phase when the magnetic field is removed ), and confirmed it experimentally in an ausaged Fe-Ni-Co-Ti alloy. It became apparent that we can control the shape memory alloy by magnetic field as well as temperature and stress.3)Control of martensite variants and giant magnetostrictionSince the finding of magnetic field-induced large strain in Ni_2MnGa, ferromagnetic shape memory alloys have been attracted great interest by many researchers. We found that when the magnetic field is applied on martensite state of Ni_2MnGa and Fe-Pd alloys along [001]_p (P is the symbol of the parent, and this direction is one of the easy axis of martensite) direction, the variants whose hard axis is orientated to the field direction is completely converted to the variant whose easy axis is orientated to the field direction. Furthermore, we discovered a new alloy whose variants can be controlled by magnetic field, i.e., Fe_3Pt single crystal. The reversible strain of Fe_3Pt is about 0.6%, which is more than three times as large as that of TERFENOL-D (practically used giant magnetostrietive material). Less